The present invention relates to a process and apparatus for forming plastic sheet. In particular, the present invention relates to a process and apparatus for forming plastic sheet having low residual stress and high surface quality. Plastic sheet formed according to the process of the present invention is particularly useful in optical and electronic display applications, such as, for example, optical windows, optical filters, recording media, and liquid crystal displays ("LCD").
Sheets of optical quality glass or quartz are used in electronic display applications as "substrates." In such applications, a "substrate" is a sheet of material used to build an electronic display. Such substrates can be transparent, translucent or opaque, but are typically transparent. In general, such sheets have conductive coatings applied thereto prior to use as substrates. Such substrates often have stringent specifications for optical clarity, flatness and minimal birefringence, and typically must have high resistance to gas and solvent permeation. Mechanical properties such as flexibility, impact resistance, hardness and scratch resistance are also important considerations. Glass or quartz sheets have been used in display applications because these materials are able to meet the optical and flatness requirements and have good thermal and chemical resistance and barrier properties; however, these materials do not have some of the desired mechanical properties, most notably low density, flexibility and impact resistance.
Because of the mechanical limitations of glass or quartz sheet in optical or display applications, it is desirable to use plastic sheet in such applications. Although plastic sheets have greater flexibility, are more resistant to breakage, and are of lighter weight than glass or quartz sheets of equal thickness, it has been very difficult to produce plastic sheet having the requisite optical specifications needed for use in optical and display applications at reasonable costs. Moreover, many types of plastic sheet undergo unacceptable dimensional distortion when subjected to substrate processing conditions during manufacture of the display devices, particularly with respect to temperature.
There are several commercially utilized methods for producing plastic sheet and film, including casting, extrusion, molding, and stretching operations. Of these methods, several are not suitable for producing high quality plastic sheet. As used throughout this specification, the term "high quality" is used to describe plastic sheet having the following characteristics: low surface roughness, low waviness, low thickness variation, and minimal amount of polymer chain orientation (for example, as measured by asymmetric physical properties, birefringence or thermal shrinkage).
For example, injection molding is likely to produce high amounts of polymer chain orientation, especially for thin sheets (i.e., 1 mm thickness or less), due to the flow of molten plastic into the mold, which unacceptably increases birefringence for polymers with non-negligible photoelasticity (stress optic) coefficients. Injection compression molding is an improved molding process which allows squeezing of the polymer after injection for the purpose of improving surface quality and reducing polymer chain orientation. However, even with these improvements, injection compression molding has limited ability to produce high quality sheet.
Compression molding and press polishing may be used to produce sheets with good surface quality; however, the squeezing flow inherent in such processes results in polymer chain orientation which results in unacceptable shrinkage during thermal cycling. Moreover, these processes are not continuously operable and therefore increase labor and production costs.
Stretching operations (for example, for the production of uniaxially- or biaxially-oriented films) and blown film extrusion inherently introduce large amounts of polymer chain orientation and are unsuited for the production of high quality plastic sheet.
Solvent casting can be used to produce high quality film; however, there are practical limitations to the maximum film thickness which can be produced by this method. In addition, the solvent used in the casting must be removed after formation of the sheet.
Sheet extrusion is run as a continuous operation, but this process introduces unacceptable polymer chain orientation due to the nature of the polymer flow in the die and between the polished rollers in the roll stack.
There is therefore a continuing need for a method for producing relatively inexpensive, high quality plastic sheet in a continuous fashion, wherein the resultant plastic sheet is capable of use as a substrate in optical and electronic display applications.